The known delay control device of FIG. 1 uses DAC (Digital to Analog Converter), and controllable current sinks for adjusting the maximum current of each inverter in the ring oscillator. The current is used to control the speed of charging and discharging of the capacitors in the inverter. The delay is approximately linear function of the invert of maximum current I defined by the current sources. This is described in document by V. De Heyn, G. Van der Plas, J. Ryckaert, J. Craninckx, “A fast start-up 3 GHz-10 GHz digitally controlled oscillator for UWB impulse radio in 90 nm CMOS,” in Proc. European Solid State Circuits Conference, pp. 484-487, 11-13 Sep. 2007.
The device of FIG. 1 is a prior art device, that is based on a constant current source, a diode mode transistor and a hank of transistors acting as adjustable resistor for tuning the bias of the current mirrors of the delay devices. The diode connected transistor is generating approximately constant voltage drop that is summed with the bank of transistors and resistor R voltage. The diode mode transistor is ensuring the current mirrors staying in active region and keeping the current at least to a minimum value that is defined by the constant current source, sizes of transistors and the minimum voltage drop over the transistor bank and the resistor.
The problem is that the minimum mirrored current is quite high, and therefore the usable maximal delay is short.
Another well known way is to use current mirror and controllable current source for controlling the delay devices load capacitance discharge current. For all digital circuit there is not enough operating voltage headroom for properly working analogue elements to generate both digitally controlled current and a current mirror.
The other well known way to control the delay is to control the load capacitance of the inverters. Load capacitance control limits the minimum delay considerably compared to delay device current limiting control, because adding load capacitance always slows down the delay device, and the capacitance is not possible to control to zero.